Medical electrodes are applied to the skin for a variety of purposes, including defibrillation, pacing, and monitoring electrical activity (e.g., ECG). A typical medical electrode has a thin metallic conductor (e.g., tin) sandwiched between an insulating backing layer and a conductive gel layer. The metallic conductor is usually cut from sheets of metal by die cutting equipment.
Electrical current flows from the metallic conductor through the gel layer to the skin. The gel layer contains an electrolyte that makes it conductive, and is typically either a nearly solid (highly viscous) gel or a liquid gel supported in a sponge-like layer. Because the gel is normally in contact with the metallic conductor throughout the life of the electrode, corrosion can naturally occur owing to variations in energy levels at the interface between the gel and metallic conductor. This corrosion is often the determining factor in setting the shelf life of the electrode.
One solution proposed for countering corrosion of the metallic conductor is to use stainless steel (U.S. Pat. No. 5,356,428), but, in fact, the corrosion still occurs with stainless steel. U.S. Pat. No. 4,419,998 discloses coating tin with stannous chloride, which helps to reduce corrosion. Putting a rubber layer between the metallic conductor and the gel has been attempted. Replacing the gel with a conductive polymer, to eliminate the salt solution, has been done, but the electrode tends to have poorer connectivity with the skin of the patient, and such conductive polymers tend to be oxygen sensitive and to degrade with age.
Another solution proposed for addressing corrosion, and thereby lengthening the shelf life of such electrodes, is disclosed in U.S. Pat. No. 6,019,877, entitled “Protecting Medical Electrodes from Corrosion”, filed on Jun. 18, 1998. A sacrificial element electrically connected to the metallic conductor and also exposed to the gel layer forms an anode-cathode cell in which the sacrificial element functions as a sacrificial anode that corrodes, protecting the metallic conductor, which functions as the cathode, from corrosion.